This invention relates to the field of force sensing transducers, such as inertial guidance accelerometers, and more particularly, to a flexure for hingedly or translationally connecting a force sensing element to a mounting base.
In the type of force transducers, such as the accelerometers disclosed in Jacobs U.S. Pat. No. 3,702,073 and Hanson U.S. Pat. Nos. 4,182,187 and 4,250,757, a force sensing element is secured to a mounting base or ring by means of a flexure which allows the element to move in response to small forces relative to the base. In such an accelerometer, the flexure may have a bifilar construction consisting of a pair of thin planar members.
In order to provide electrical connections to components located on the sensing element, flexible leads between the base and sensing element may be used, or a thin film of conductive material may be deposited directly on the flexure or on a nonconductive coating on the flexure if the flexure itself is electrically conductive. When such materials are deposited on the flexure, stresses are set up in the flexure due to the differing temperature coefficients of the flexure and conductive materials, or by the deposition process itself. These stresses in turn result in forces which attempt to deflect the sensing element from a neutral position. In servo-loop transducers which apply to restoring force to maintain the sensing element in the neutral position, bias error is developed as a result of these stresses. In open-loop transducers where the amount of deflection of the sensing element is measured, bias error is also produced.
In those transducers which utilize conductive coatings, an effort is made to cancel out the film stresses by depositing the films equally on the upper and lower faces of the flexure sections. While this construction reduces errors to some degree, it requires a precise balancing during the deposition process so that the film thickness is equal on both sides of the flexure section. Moreover, this balancing is dependent upon film stress stability with respect to time, and is also dependent upon other factors such as ambient temperature, material purity and surface contamination.
In general, in prior transducers it has been found desirable to utilize the thinnest possible flexure consonant with strength and elasticity requirements for proper operation, so that stress effects leading to bias errors are minimized. However, it has been found that the spring rate, whether angular or linear, provided by a flexure is proportional to the cube of the thickness "t" thereof, while the bending moment of the flexure due to stress caused by deposition of the conductive strips is only proportional to t. For example, if the thickness of the flexure is reduced by 30% such that the angular spring rate provided thereby is changed from 20.sup.g /radian to 7.sup.g /radian, the error moment due to stress effects in the conductive plating is reduced by a factor of only 1.42. Hence, it can be seen that the lower limit of the range of acceptable spring rates provided by a conventional flexure will be reached well before the error moment is reduced to an insignificant value. Consequently, for these types of flexures, trade-offs must be made between obtaining the desired spring rate and flexure strength and minimizing stress effects which lead to errors.